Replying to the heaps of forum feedback saying "this looks terrible" on Bethesda's official Fallout 3 forum, Todd Howard noted (by proxy):<blockquote>I agree, and I don't think the cover shows him well. It all has to do with the FOV of the character when he got rendered for the cover image (it's not in-game), it makes his head look huge and his torso look skinny. I assure you in-game, the armor looks much bulkier. See the other screenshots we released. Also, this power armor is specifically not the T-51b.</blockquote>And there's the confirmation that it's not T-51b, though most people had assumed as much by now.
Link: Gstaff forum post on BGSF.
Link: Gstaff forum post on BGSF.